C++标准模板（STL）- 概念库 (C++20) - 指定能复制构造和移动构造一个类型的对象

概念库提供基础语言概念的定义，它们能用于进行模板实参的编译时校验，以及基于类型属性的函数派发。这些概念在程序中提供等式推理的基础。

标准库中的大多数概念一同加上了语法及语义要求。通常，编译器只能检查语法要求。若在使用点语义要求未得到满足，则程序为病式，不要求诊断。

类型支持（基本类型、RTTI、类型特性）

指定能复制构造和移动构造一个类型的对象

std::copy_constructible

定义于头文件 `<concepts>`
template <class T> concept copy_constructible = std::move_constructible<T> && std::constructible_from<T, T&> && std::convertible_to<T&, T> && std::constructible_from<T, const T&> && std::convertible_to<const T&, T> && std::constructible_from<T, const T> && std::convertible_to<const T, T>;		(C++20 起)

概念 copy_constructible 若符合这些条件则得到满足： T 为左值引用类型，或若它是 move_constructible 对象类型，而能从（可为 const 的）该类型左值或 const 右值，在直接和复制初始化语境中以通常语义构造该类型对象（构造副本而不更改源）。

更精确而言，若 T 为对象类型，则 copy_constructible<T> 仅若符合下列条件才得到满足。给定

v ， T 类型左值（可为 const ）或为 const T 类型右值，

下列为真：

定义 T u = v; 后， u 等于 v ；
T(v) 等于 v 。

参阅

is_copy_constructible

is_trivially_copy_constructible

is_nothrow_copy_constructible

(C++11)

检查类型是否拥有复制构造函数
(类模板)

调用示例

#include <iostream>
#include <type_traits>class E
{
public:template<class T> E(T&&) { }
};class A {};
class B : public A {};
class C {};
class D
{
public:operator C(){return c;}  C c;
};//自定义的实现
namespace std
{template< class T, class... Args >
const bool is_constructible_v = is_constructible<T, Args...>::value;template< class T, class... Args >
const bool is_trivially_constructible_v = is_trivially_constructible<T, Args...>::value;template< class T, class... Args >
const bool is_nothrow_constructible_v = is_nothrow_constructible<T, Args...>::value;template< class T >
const bool is_nothrow_destructible_v = is_nothrow_destructible<T>::value;template < class T >
const bool destructible = is_nothrow_destructible<T>::value;template <class From, class To>
constexpr bool convertible_to = (std::is_convertible<From, To>::value);template < class T, class... Args >
const bool constructible_from = (destructible<T> && is_constructible_v<T, Args...>);template< class T >
const bool move_constructible = (constructible_from<T, T> && convertible_to<T, T>);template <class T>
const bool copy_constructible =move_constructible<T> &&constructible_from<T, T&> && convertible_to<T&, T> &&constructible_from<T, const T&> && convertible_to<const T&, T> &&constructible_from<T, const T> && convertible_to<const T, T>;
}
int main()
{std::cout << std::boolalpha;std::cout << "std::is_convertible<B*, A*>::value:           "<< std::is_convertible<B*, A*>::value << std::endl;std::cout << "std::is_convertible<A*, B*>::value:           "<< std::is_convertible<A*, B*>::value << std::endl;std::cout << "std::is_convertible<B*, C*>::value:           "<< std::is_convertible<B*, C*>::value << std::endl;std::cout << "std::is_convertible<D, C>::value:             "<< std::is_convertible<D, C>::value << std::endl;std::cout << "std::is_convertible<E, A>::value:             "<< std::is_convertible<E, A>::value << std::endl;std::cout << "std::is_convertible<int, double>::value:      "<< std::is_convertible<int, double>::value << std::endl;std::cout << "std::is_convertible<int, char>::value:        "<< std::is_convertible<int, char>::value << std::endl;std::cout << "std::is_convertible<std::string, char>::value:"<< std::is_convertible<std::string, char>::value << std::endl;std::cout << "-----------------------------------------------" << std::endl;std::cout << std::endl;std::cout << "std::destructible<A>:                         "<< std::destructible<A> << std::endl;std::cout << "std::destructible<B>:                         "<< std::destructible<B> << std::endl;std::cout << "std::destructible<C>:                         "<< std::destructible<C> << std::endl;std::cout << "std::destructible<D>:                         "<< std::destructible<D> << std::endl;std::cout << "std::destructible<E>:                         "<< std::destructible<E> << std::endl;std::cout << "std::destructible<float>:                     "<< std::destructible<float> << std::endl;std::cout << "std::destructible<int>:                       "<< std::destructible<int> << std::endl;std::cout << "std::destructible<bool>:                      "<< std::destructible<bool> << std::endl;std::cout << std::endl;std::cout << "-----------------------------------------------" << std::endl;std::cout << std::endl;std::cout << "std::constructible_from<B, A>:                "<< std::constructible_from<B, A> << std::endl;std::cout << "std::constructible_from<A, B>:                "<< std::constructible_from<A, B> << std::endl;std::cout << "std::constructible_from<B*, A*>:              "<< std::constructible_from<B*, A*> << std::endl;std::cout << "std::constructible_from<A*, B*>:              "<< std::constructible_from<A*, B*> << std::endl;std::cout << "std::constructible_from<B*, C*>:              "<< std::constructible_from<B*, C*> << std::endl;std::cout << "std::constructible_from<D, C>:                "<< std::constructible_from<D, C> << std::endl;std::cout << "std::constructible_from<E, A>:                "<< std::constructible_from<E, A> << std::endl;std::cout << "std::constructible_from<int, double>:         "<< std::constructible_from<int, double> << std::endl;std::cout << "std::constructible_from<int, char>:           "<< std::constructible_from<int, char> << std::endl;std::cout << "std::constructible_from<std::string, char>:   "<< std::constructible_from<std::string, char> << std::endl;std::cout << "-----------------------------------------------" << std::endl;std::cout << std::endl;std::cout << "std::convertible_to<B, A>:                    "<< std::convertible_to<B, A> << std::endl;std::cout << "std::convertible_to<A, B>:                    "<< std::convertible_to<A, B> << std::endl;std::cout << "std::convertible_to<B*, A*>:                  "<< std::convertible_to<B*, A*> << std::endl;std::cout << "std::convertible_to<A*, B*>:                  "<< std::convertible_to<A*, B*> << std::endl;std::cout << "std::convertible_to<B*, C*>:                  "<< std::convertible_to<B*, C*> << std::endl;std::cout << "std::convertible_to<D, C>:                    "<< std::convertible_to<D, C> << std::endl;std::cout << "std::convertible_to<E, A>:                    "<< std::convertible_to<E, A> << std::endl;std::cout << "std::convertible_to<int, double>:             "<< std::convertible_to<int, double> << std::endl;std::cout << "std::convertible_to<int, char>:               "<< std::convertible_to<int, char> << std::endl;std::cout << "std::convertible_to<std::string, char>:       "<< std::convertible_to<std::string, char> << std::endl;std::cout << "-----------------------------------------------" << std::endl;std::cout << std::endl;std::cout << "std::move_constructible<B>:                   "<< std::move_constructible<B> << std::endl;std::cout << "std::move_constructible<A>:                   "<< std::move_constructible<A> << std::endl;std::cout << "std::move_constructible<B*>:                  "<< std::move_constructible<B*> << std::endl;std::cout << "std::move_constructible<A*>:                  "<< std::move_constructible<A*> << std::endl;std::cout << "std::move_constructible<C*>:                  "<< std::move_constructible<C*> << std::endl;std::cout << "std::move_constructible<D>:                   "<< std::move_constructible<D> << std::endl;std::cout << "std::move_constructible<E>:                   "<< std::move_constructible<E> << std::endl;std::cout << "std::move_constructible<int>:                 "<< std::move_constructible<int> << std::endl;std::cout << "std::move_constructible<char>:                "<< std::move_constructible<char> << std::endl;std::cout << "std::move_constructible<std::string>:         "<< std::move_constructible<std::string> << std::endl;std::cout << "-----------------------------------------------" << std::endl;std::cout << std::endl;std::cout << "std::copy_constructible<B>:                   "<< std::copy_constructible<B> << std::endl;std::cout << "std::copy_constructible<A>:                   "<< std::copy_constructible<A> << std::endl;std::cout << "std::copy_constructible<B*>:                  "<< std::copy_constructible<B*> << std::endl;std::cout << "std::copy_constructible<A*>:                  "<< std::copy_constructible<A*> << std::endl;std::cout << "std::copy_constructible<C*>:                  "<< std::copy_constructible<C*> << std::endl;std::cout << "std::copy_constructible<D>:                   "<< std::copy_constructible<D> << std::endl;std::cout << "std::copy_constructible<E>:                   "<< std::copy_constructible<E> << std::endl;std::cout << "std::copy_constructible<int>:                 "<< std::copy_constructible<int> << std::endl;std::cout << "std::copy_constructible<char>:                "<< std::copy_constructible<char> << std::endl;std::cout << "std::copy_constructible<std::string>:         "<< std::copy_constructible<std::string> << std::endl;return 0;
}

输出

std::is_convertible<B*, A*>::value:           true
std::is_convertible<A*, B*>::value:           false
std::is_convertible<B*, C*>::value:           false
std::is_convertible<D, C>::value:             true
std::is_convertible<E, A>::value:             false
std::is_convertible<int, double>::value:      true
std::is_convertible<int, char>::value:        true
std::is_convertible<std::string, char>::value:false
-----------------------------------------------std::destructible<A>:                         true
std::destructible<B>:                         true
std::destructible<C>:                         true
std::destructible<D>:                         true
std::destructible<E>:                         true
std::destructible<float>:                     true
std::destructible<int>:                       true
std::destructible<bool>:                      true-----------------------------------------------std::constructible_from<B, A>:                false
std::constructible_from<A, B>:                true
std::constructible_from<B*, A*>:              false
std::constructible_from<A*, B*>:              true
std::constructible_from<B*, C*>:              false
std::constructible_from<D, C>:                false
std::constructible_from<E, A>:                true
std::constructible_from<int, double>:         true
std::constructible_from<int, char>:           true
std::constructible_from<std::string, char>:   false
-----------------------------------------------std::convertible_to<B, A>:                    true
std::convertible_to<A, B>:                    false
std::convertible_to<B*, A*>:                  true
std::convertible_to<A*, B*>:                  false
std::convertible_to<B*, C*>:                  false
std::convertible_to<D, C>:                    true
std::convertible_to<E, A>:                    false
std::convertible_to<int, double>:             true
std::convertible_to<int, char>:               true
std::convertible_to<std::string, char>:       false
-----------------------------------------------std::move_constructible<B>:                   true
std::move_constructible<A>:                   true
std::move_constructible<B*>:                  true
std::move_constructible<A*>:                  true
std::move_constructible<C*>:                  true
std::move_constructible<D>:                   true
std::move_constructible<E>:                   true
std::move_constructible<int>:                 true
std::move_constructible<char>:                true
std::move_constructible<std::string>:         true
-----------------------------------------------std::copy_constructible<B>:                   true
std::copy_constructible<A>:                   true
std::copy_constructible<B*>:                  true
std::copy_constructible<A*>:                  true
std::copy_constructible<C*>:                  true
std::copy_constructible<D>:                   true
std::copy_constructible<E>:                   true
std::copy_constructible<int>:                 true
std::copy_constructible<char>:                true
std::copy_constructible<std::string>:         true